Start-up system for once through boilers



Oct. 14, 1969 G. F. FOLDES 3,472,207

START-UP SYSTEM FOR ONCE THROUGH BOILERS Original Filed May 26, 1966 I D 2 I Hob United States Patent 3,472,207 START-UP SYSTEM FOR ONCE THROUGH BOILERS Gabriel F. Foldes, Banstead, England, assignor to Foster Wheeler Corporation, Livingston, N.J., a corporation of New York Continuation of application Ser. No. 553,191, May 26,

1966. This application June 20, 1968, Ser. No. 752,415

Int. Cl. F22d 7/00; F22b 35/10 US. Cl. 122-406 11 Claims ABSTRACT OF THE DISCLOSURE A once-through vapor generator including a start-up system for recycling a variable portion of heated fluid such that there is sufficient flow to prevent tube bum-out. A pressure reduction throttling system is provided between the steam generating section and primary superheater to reduce the pressure differential in the recirculation section.

CROSS REFERENCE TO COPENDING APPLICATION This application is a continuation of my application Ser. No. 553,191, filed May 26, 1966, now abandoned, for Start-Up System for Once-Through Boilers.

In this specification and claims the terms water and steam will be used. It is to be understood, however, that these terms are intended to include references to any liquid and its vapour unless the context specifically requires otherwise.

In steam generators it is important to ensure that at all times there is sufficient flow through the tubes to keep them cool enough to prevent burn-out. In the form of boiler in which the tubes are connected between inlet and outlet headers there is generally no difficulty in arranging for a natural or forced circulation which ensures, except in the case of a failure of the water supply, that water or a steam and water mixture is always passing through the tubes at a suflicient rate to prevent burnouts.

In the case of a once-through boiler, however, there is no re-circulation and it can be diflicult to maintain th safe minimum flow through the highly-heated tubes during start-up or at low loads since the flow rate through the tubes is proportional to the steam requirements.

The diflieulty could be overcome at least at low loads by always maintaining the safe minimum flow through those tubes and sending to waste any excess steam, but the disadvantage of this is a large drop in efliciency of the cycle through the waste of heat.

In accordance with the invention provision is made for the re-circulation through at least the highest heat absorption tubes of a variable fraction of the flow, the remainder thereof being passed through a separator, the steam from which is passed to the superheater and the water to the feed water supply. The fraction is chosen so that both the steam generator tubes and the superheater tubes are safeguarded against being burnt-out.

During start-up or at low loads the steam generator tubes and the superheater tubes are safeguarded and once the steam generator is working at sufficiently high loads the separator can be shut off from the system and steam fed directly from the steam generator tubes to the superheater. Also the recirculation through the steam generating tubes can be stopped.

The re-circulation need only be elfected through the steam generating tubes which are subjected to the most heating and allowance can be made for this in accordance with the invention. The re-circulation can if desired also be extended around at least part of the superheater. The re-circulation is not normally effected through the economiser since its tubes are not usually subjected to the danger of a burn-out.

The heated fluid may be re-circulated. by a pump in a re-circulating line. Preferably the hot-re-circulated fluid is not fed directly to the re-circulating pump, but is first mixed with a proportion of cooler water from the economiser section. In this Way the pump does not have to deal with the very hot fluid from the steam generating tubes which leads to simplification in the choice and design of this pump, and also reduce the cost of the pump. Another advantage of the invention is that during normal operation of the boiler this pump will be inoperative and provided it is cut-01f from the boiler by valves and suitable provisions are made it can be serviced while the boiler is operating.

The separator may have a relatively small capacity since it is only used to handle the steam requirements at low loads. By using such a separator, which is preferably a flash evaporator, a supply of steam is very quickly provided during start-up, and this steam is used to protect the superheater tubes. This steam after being superheated can be fed to the turbine so as to start heating it gently and to start it rolling, and after this steam has passed through the turbine once it can be sent to the reheater tubes to protect them. Some of the steam either before or after superheating can be fed to turbine seals and deaerator, bringing them in operation very soon after start-up and thus reducing the chances of contamination of the feed water by oxygen. It will only be after all of the steam requirements have been met that any heat will have to be wasted by condensing excess steam.

The invention may be applied to steam generators operating at subor super-critical pressures.

Embodiments of the invention will now be described, by way of example, with reference to the accompanying diagrammatic drawing, in which:

FIGURE 1 is a flow sheet of a oncethrough steam generator in accordance with the invention; and

FIGURE 2 is a flow sheet of a part of a modified oncethrough steam generator.

The once-through steam generator has an economiser section 1, an evaporator section 2, a superheater section 3 and a reheater section '4, all these sections being constituted by banks of tubes and tubes lining the walls of the furnace and enclosures in the usual manner for a steam generator.

During normal operation of the generator when there is sutficient flow of water and steam to protect the tubes of the boiler, feed water is fed by a feed pump 5 serially through the economiser section 1, the evaporator section 2, a valve 6 being open, and the superheater section 3, a valve 7 being open and valves 8, 9 and 10 being closed, and from there the superheated steam is led to a turbine 11, a valve 12 being open. Exhaust steam from the superheater side of the turbine is taken back to the generator and passed through the reheater section 4 before being led back to the turbine and from there to a condenser 13.

During start-up and at low loads of the generator only a small amount of steam is necessary and in the oncethrough generator this means that the feed pump 5 has only to supply a small amount of water. In order however, to protect the evaporator tubes the valve 6 is shut, the valve 10 is opened and heated fluid is re-circulated through the line 10a by a pump 14, through the evaporator section 2, valves 15 being open. It is not always essential that heated fluid be re-circulated through all parts of the evaporator section 2 and if desired the system may be designed so that re-circulation takes place only through the greatest heat absorption tubes of this section.

It will be noticed that the pump 14 is not in the recirculation line a. Instead re-circulated fluid is first mixed with water from the economiser section 1 and then passed through the pump 14. This means that the pump does not have to handle water of such a high temperature that the choice and design of the pump is difficult. Consequently, a relatively inexpensive pump can be used.

Also at low loads or during a start-up the valve 7 will be closed and the valve 8 will be open. Then the heated fluid from the evaporator section which is not re-circulated by the pump 14 is led to a steam separator 16. From this separator water is led to the condenser 13, while steam can be fed to the superheater section 3 if the valve 9 is open or it can by-pass the section 3 by opening a valve 17.

During start-up at low loads valves 9 and 17 will be controlled so as to send sufficient steam to the superheater section 3 to keep this cool. By varying the steam flow and pressure through the superheater section 3 and also by manipulating the firing rate within limits, the steam temperature can be regulated to suit the requirements of the turbine during the start-up period. After passing through the superheater section, the superheated steam will either be sent to the turbine to start or keep this rolling and any excess superheated steam will be led to a valve 18 and mixed with any excess steam from the steam separator 16 and passing through the valve 17. The steam passing through either or both of the valves 17 and 18 is led to a de-aerator 19, to the turbine seals at 24 and to a high-pressure feed water heater 20 so as to recover heat from the steam, water condensed in this heater 20 being returned to the feed water in the deaerator 19. If after all the above demands for steam have been met there is still some excess, this can be passed through a valve 21 to a de-superheater 22 and then to the condenser 13. This recourse will not, however, usually be necessary.

The turbine steam pressure during start-up and low load is regulated by adjustment of the valve 8 in combination with the valves 12 and 18.

Once the generator reaches a normal load the recycling by the pump 14 can be stopped and the heated fluid will then pass normally through the various sections of the generator again in series. There can be some removal of steam, unsuperheated steam through the valves 9 and 17, superheated steam through the valve 18 or a mixture of both, for leading to the turbine seals and the de-aerator, although generally, once the turbine is working, the deaerator will be supplied with extraction steam from the main or any auxiliary turbine.

The exact working conditions under which the recycling is started and the steam separator is brought into effect will depend upon the particular steam generator under consideration. Generally, however, it is considered important to maintain a flow of at least about 30% continuous maximum rating through the highest heat absorption tubes to prevent a burn-out. Therefore the recycling pump should be arranged to maintain this flow even when the load is less than 30% continuous maximum rating and preferably the pump should give an approximate flow of 60% continuous maximum rating through the tubes to be protected. The actual recycling rate is achieved by the characteristics of the pump itself and by feed regulating valves. As a safeguard, however, a temperature control 25 is provided so that if the fluid temperature fed to the pump 14 exceeds a predetermined value, the feed water regulator valve 26 and the feed pump 5 are adjusted to increase the rate of flow of feed water.

The steam separator is preferably brought into operation at below about 30% continuous maximum rating so as to give a clean steam for the superheater and because of this it need be sized to handle no more than this load of steam. This allows it to be relatively cheap and simple.

During normal operation of the boiler the pump 14 is isolated by the valves 15. It can therefore be dismantled and serviced during boiler operation. If desired a pair of pumps 14 may be employed in parallel and then even if one fails it is still possible to start-up and operate the boiler normally at low loads.

The separator 16 is connected upstream of the superheater section 3. Also the separator and associated pipework do not have to handle such high temperature fluid as they would if the separator was positioned downstream of the superheater.

Whether recirculation is effected around all of the evaporator section 2 or only just a part of it will depend upon the boiler arrangement and which tubes in the section are subjected to the highest heat absorption rates. If convenient the recirculation may be applied around only part of the section 2 and then fluid will be recirculated through line 10c to the line 10a.

FIGURE 2 shows a modification of part of the steam generator.

The superheater section comprises a primary superheater section 3A and a secondary superheater section 3B. Between the primary section 3A and the steam generating section 2 are pressure reducing or throttling valves 23. The fluid recirculated is taken by line 10b to the line 10a so as not to place too great a pressure differential across the pump 14. This arrangement has the advantage that the section 1 and the pump 14 which are upstream of the valves 23 can be brought up to full operating pressure while the turbine can still be supplied with low pressure high enthalpy steam according to its requirements during start-up. This arrangement provides even greater flexibility of matching the turbine requirements during start-up.

In this embodiment the furnace tubes steam pressure and the turbine steam pressure can be regulated separate- 1y. The furnace tubes steam pressure can be regulated by the valves 23 while the turbine steam pressure is controlled by adjustment of the valve 8 in combination with valves 12 and 18 during the various stages of the start-up.

The reducing valve 8 is not subjected to extremely heavy duty since, in the case of the embodiment of FIG- URE 1, it is not subjected to full boiler operating pressure and, in the case of the embodiment of FIGURE 2, it is again not subjected to full boiler operating pressure because of the valves 23'.

What is claimed is:

1. In a once-through boiler comprising a steam generating section, means for supplying fluid to be heated in said steam generating section and a superheating section to which heated fluid is fed from said steam generating section during normal operation of said boiler, a start-up and low load system comprising, independent recirculating means connected at the cooler entry of the fluid in the steam generator outside the heated fluid feeding system but in fluidic association therewith for force recycling 3. variable proportion of heated fluid around at least part of said steam generating section, a recirculating pump for force circulating fluid from the cooler fluid entry of said steam generating system, temperature control means operable for supplying an increase of fluid flow from the fluid supply so that the temperature of the fluid fed to the recirculating pump is retained at a predetermined value for protecting said recirculating pump against high temperature fluid, separator means to which is fed the remainder of said heated fluid from said steam generating section for separation into water and steam, and means for feeding said steam from said separator means to said superheating section, said proportion of heated fluid recycled around said steam generating section being chosen so as to maintain suflicient flow of fluid therein to prevent tube burn-out.

2. The boiler of claim 1 comprising conduit means outside the heated fluid feeding system and said recirculating means but in fluidic association therewith for recycling said proportion of heated fluid, means for mixing recycled fluid with said fluid to be heated in said steam generating section, and means for supplying the mixture of said recycled fluid and said fluid to be heated to said steam generating section.

3. In a once-through steam boiler as claimed in claim 1, further comprising a plurality of conduits connected in parallel at the center fluid entry in said steam generator section, one of said conduits directing fluid through the main fluid flow of the boiler, another of said conduits operably connected for recirculatory fluid through a predetermined portion of said generating section and a further conduit independent from said prior mentioned conduit, but in fluidic connection therewith, including a pair of serially connected valves interposed around said recirculating pump for force circulating fluid from the cooler fluid entry through the predetermined portion of said steam generating system.

4. The structure of claim 1 further comprising a fluid regulator valve for regulating the fluid supply, and said temperature control means connected at the entry of fluid to said recirculating pump to control said regulator valve for supplying an increase of fluid flow upon the fluid entering said pump exceeding a predetermined temperature.

5. The boiler of claim 1 further comprising throttling means to which is fed the remainder of said heated fluid, whereby said steam generating section can be operated at a higher pressure than said superheater.

6. In a once-through steam boiler as claimed in claim 1 further comprising, a primary superheater section, means for feeding fluid heated in said steam generating section to said primary superheater, and a secondary superheater to which during normal operation of said boiler heated fluid from said primary superheater fluid is fed.

7. The boiler of claim 6 further comprising throttling means between said steam generating section and said primary superheater, whereby during start-up and low load said steam generator can be brought to normal operating pressure while allowing lower pressure vapour to be supplied to said primary and secondary superheaters.

8. The boiler of claim 6, comprising conduit means for recycling said proportion of heated fluid and mixing it with said fluid to be heated in said steam generating section, and pump means for feeding this mixture of fluids to said steam generating section.

9. The boiler of claim 6 in which said separator is a flask evaporator.

10. A once-through vapor generator comprising a steam generating section, feed pump means for feeding fluid to the section to be heated therein, a primary superheater section, means for feeding fluid heated in said steam generating section to said primary superheater section, a secondary superheater section to which heated fluid from said primary superheater section is fed during normal operation of said vapor generator, recirculating means connected at the cooler entry of the fluid in said steam generating section but in fluidic association therewith for recycling a variable portion of heated fluid from said steam generating section around at least part of said steam generating section so that there is suflicient flow through the steam generating section to prevent tube burn-out during start-up and the remainder of heated fluid from said steam generating section being fed to said primary superheater, a recirculating pump for force circulating fluid from the cooler fluid entry of said steam generating sys tem, temperature control means for sensing the temperature of the fluid entering said recirculating pump, and a feed water regulator valve operably connected to said temperature control means for adjusting the rate of flow of feed water to prevent the temperature of the fluid fed to said recirculating pump from exceeding a predetermined value, separator means to which the fluid from said primary superheater is fed for separation into liquid and vapor, and means for feeding vapor from said separator means to said secondary superheater, and pressure reduction throttling means between said steam generating section and said primary superheater, whereby during startup and low load said steam generating section can be brought to normal operating pressure while allowing lower pressure vapor to be supplied to said primary and secondary superheater sections.

11. A vapor generating system for operation in the supercritical pressure range during start-up, comprising a steam generating section, feed pump means for feeding fluid to the section to be heated therein, a primary superheater section, means for feeding fluid heated in said steam generating section to said primary superheater section, and a secondary superheater section to which heated fluid from said primary superheater section is fed during normal operation of said vapor generating system, turbine means, recirculating means including conduits connected in parallel fluidic association with the fluid to be heated within said steam generating section for recycling a variable proportion of heated fluid from said. steam generating section around at least a part of said steam generating section so that there is sumcient flow through the steam generating section to prevent tube burn-out and the remainder of heated fluid from said generating section being fed to said primary superheater section, said conduits serving to mix said portion of heated fluid with said fluid to be heated in said steam generating section, a recirculating pump for force circulating fluid from the cooler fluid entry of said steam generating system, temperature control means for sensing the temperature of the fluid entering said recirculating pump, and a feed water regulator valve operably connected to said temperature control means for adjusting the rate of flow of feed water to prevent the temperature of the fluid fed to said recirculating pump from exceeding a predetermined value, separator means to which the fluid from said primary superheater is fed for separation into liquid and vapor, and means for feeding vapor from said separator means to said secondary superheater section, pressure reduction throttling means between said steam generating section and said primary superheater section, whereby during start-up and low load said vapor generating system can be brought to normal operating pressure while allowing lower pressure vapor to be supplied to said primary and secondary superheater sections.

References Cited UNITED STATES PATENTS 3,135,096 6/ 1964 Schroedter. 3,183,896 5/1965 Lytle et a1. 3,194,217 7/1965 Grabowski. 3,194,218 7/1965 Schroedter et al.

CHARLES J. MYHRE, Primary Examiner 

